1. Field of the Invention
The present invention relates generally to that area of metrologic technology concerned with measuring a surface's topology, and, more particularly, to profilometry, and to atomic force microscopy ("AFM), also sometimes referred to as scanning force microscopy ("SFM").
2. Description of the Prior Art
Recently, the field of surface profilometry has expanded greatly. In addition to advances in classical profilometry, the nascent fields of tunneling force microscopy and AFM have greatly enlarged the interest, scope and capability of profilometric technology.
Classical profilometry scans a surface along orthogonal X-axis and Y-axis directions using a diamond tipped stylus while measuring the stylus' vertical (Z-axis) displacement. In many commercial instruments, the stylus is connected to a linear variable differential transformer ("LVDT") sensor, or to a capacitive plate, for sensing the stylus' vertical displacement. Typically, the stylus includes an elongated bar that is secured with a pair of coaxial pivots, while the other end of the stylus is coupled to the Z-axis displacement sensing mechanism, e.g. either a capacitor's plate for a capacitive sensor, or a ferromagnetic plunger of the LVDT sensor.
Very sensitive flexure pivot assemblies are commonly used for supporting the stylus used for classical profilometry. The components of such a flexure pivot assembly are small, delicate, require precision assembly, and therefore are expensive to manufacture. In addition, machining such stylus assemblies from discrete components tends to make them comparatively large, and the sensing elements to which they couple are also relatively large. Therefore, profilometer heads including the stylus are, in general, larger than desirable. Consequently, profilometer heads generally respond slowly to vertical displacements, and the scanning speed at which profilometers operate is limited by the inertia of the profilometer's head. Hence, improving profilometer performance while concurrently reducing their manufacturing cost and contact force makes gentler, smaller, lighter, faster and less expensive profiling heads very desirable.
The more recently developing field of AFM for measuring a surface's topography generally uses a very light, micromachined, bendable cantilever probe having a sharp tip for sensing a surface's topology at atomic dimensions. However, systems for detecting minute vertical displacement of an AFM's sensing probe, e.g. optical-beam-deflection or optical interferometry, are, in general, much larger than the cantilever itself. Consequently, it is generally difficult to move an AFM's head assembly as swiftly as desired for high speed scanning. Traditionally, AFM systems circumvented this problem by holding the sensing head assembly stationary while moving the sample along orthogonal X and Y axes. Although such a system may move small samples easily during AFM scanning, it is generally unsuited for use on large samples, such as semiconductor wafers or magnetic recording disks measuring several inches in diameter.
Accordingly, not only does AFM necessarily require a physically small AFM sensing probe, but advancing AFM technology and performance also makes a correspondingly small, light weight, and compact sensor for detecting AFM probe Z-axis displacement desirable. Integration of a compact vertical displacement sensor into an AFM probe would yield a small, light weight, and compact AFM head having a low mass. Such a low mass AFM probe would permit very high speed scanning along orthogonal X-axis and Y-axis directions by a small and compact X-axis and Y-axis drives.
Referring now to FIG. 1, depicted there is a prior art AFM or profilometer system referred to by the general reference character 20. The system 20 includes a XY axes drive 22 upon which rests a sample 24. The XY axes drive 22 scans the sample 24 laterally with respect to a sensing head 26 along a X-axis 32 and a Y-axis 34 that are orthogonal to each other, or along any other arbitrary axes obtained by compound motion along the X-axis 32 and the Y-axis 34. In the instance of an AFM, to provide rapid movement along the axes 32 and 34 the XY axes drive 22 may be provided by a piezo electric tube having 4 quadrant electrodes. As the XY axes drive 22 moves the sample 24 laterally, a probe tip or stylus 36 lightly contacts an upper surface 38 of the sample 24 while moving up and down vertically parallel to a Z-axis 42 in response to the topology of the upper surface 38. In the illustration of FIG. 1, the probe tip or stylus 36 is secured to a distal end of an elongated cantilever arm 44 extending outward from the sensing head 26. The sensing head 26, which may if necessary be servoed up and down parallel to the Z-axis 42, senses vertical deflection of the probe tip or stylus 36 by the topology of the sample's upper surface 38. A signal transmitted from the sensing head 26 to some type of signal processing device permits recording and/or displaying the topology of the upper surface 38 as detected by the system 20.
AFM applications of systems such as of the system 20 experience substantial cross coupling among movements along the mutually perpendicular axes 32, 34, and 42. Consequently, movement of the sample 24 with respect to the AFM sensing head 26, and frequently even the measurement of such movement, are insufficiently precise for metrologic applications. Consequently, at present AFM performance may be adequate for imaging, but not for metrology. The mass of the sample 24 itself (such as an 8 inch diameter semiconductor wafer) impedes high speed, precise movement of the sample 24. Therefore, scanning a massive sample 24 swiftly requires holding the sample 24 fixed while scanning the sensing head 26.
FIG. 2 depicts an alternative embodiment, prior art AFM or profilometer system. Those elements depicted in FIG. 2 that are common to the AFM or profilometer system depicted in FIG. 1 carry the same reference numeral distinguished by a prime (') designation. In the system 20' depicted in FIG. 2, the sample 24' rests on a base plate 48 which also supports a XY stage 52. In scanning the sample 241 using the system 20', the XY stage 52 moves the sensing head 26' carrying the cantilever arm 44' parallel to the orthogonal X-axis 32' and Y-axis 34', or along any other arbitrary axes obtained by compound motion along the X-axis 32' and the Y-axis 34'.
E. Clayton Teague, et al., in a technical article entitled "Para-flex Stage for Microtopographic Mapping" published the January 1988, issue of the Review of Scientific Instruments, vol. 59 at pp. 67-73 ("the Teague et al. article"), reports development of a monolithic, Para-flex XY stage 52, that the article describes as being machined out of metal. The embodiments of the monolithic plate of such an XY stage 52 is depicted both in FIGS. 3a and 3b. The XY stage 52 depicted in both FIGs. includes an outer base 62 that is fixed with respect to the system 20'. The outer base 62 is coupled to and supports a Y-axis stage 64 by means of four stage suspensions 66. Each of the stage suspensions 66 consists of an intermediate bar 68, one end of which is coupled to the outer base 62 by a flexure 72, and another, distal end of the intermediate bar 68 is coupled to the Y-axis stage 64 by a second symmetrical flexure 72. Similar to the coupling of the outer base 62 to the Y-axis stage 64, the Y-axis stage 64 is coupled to and supports a X-axis stage 74 by means of four stage suspensions 66 that are identical to the stage suspensions 66 which couple the outer base 62 to the Y-axis stage 64. The stage suspensions 66 coupling the outer base 62 to the Y-axis stage 64 and the stage suspensions 66 coupling the Y-axis stage 64 to the X-axis stage 74 are oriented perpendicular to each other. Consequently, the inner X-axis stage 74 moves substantially perpendicularly to movement of the Y-axis stage 64, with both stages 64 and 74 moving with great accuracy with respect to the outer base 62. Movement of the stages 64 and 74 with respect to the outer base 62 is effected by a pair of mutually orthogonal stepping-motor-controlled micrometer screw drives, not illustrated in any of the FIGs., which respectively have a pushrod connection to the Y-axis stage 64 and the X-axis stage 74. The screw drives extend from outside the outer base 62 through drive apertures 76 to respectively contact the Y-axis stage 64 and the X-axis stage 74. The XY stage 52 depicted in FIG. 3b differs from that depicted in FIG. 3a in that the stage suspensions 66 coupling the Y-axis stage 64 to the X-axis stage 74 are folded which reduces the space occupied by the XY stage 52. The XY stage 52 reported by C. Teague, et al. provides accurate movement along mutually perpendicular axes 32' and 34'. However, the XY stage 52 depicted in FIGS. 3a and 3b provides no motion amplification.
FIG. 4 depicts the flexure 72 indicated on the XY stage 52 depicted in FIG. 3b. The flexure 72 employs a pair of webs 82 arranged in a W-shaped configuration to span between the outer base 62 and the intermediate bar 68, between the intermediate bar 68 and the Y-axis stage 64, between the Y-axis stage 64 and the intermediate bar 68, and between the intermediate bar 68 and the X-axis stage 74. The flexure 72 depicted in FIG. 4 permits both longitudinal stretching and rotation.
If the XY stage 52 is made by conventional techniques, even a monolithic XY stage 52 such as that disclosed in the Teague et al. article, the resonance frequency is typically a few hundred Hz. Stepping-motor-controlled micrometer screw drives or other forms of push rods for displacing the XY stage 52 are typically limited to relatively low frequency operation. Consequently, the XY stage 52 of an AFM can only be servoed at relatively low speed.
Recent advances in reactive ion etching processes and apparatus for etching silicon permit forming deep vertical structures. For example the new Alcatel etcher produces etching aspect ratios of 300/1, and therefore permits etching through wafers several hundred microns thick. Other etchers having similar performance are now available. Some techniques for wet etching, (such as [100] orientation etching), may also be used to fabricate structures having correspondingly high aspect ratios. These improved processes permit construction of structures of metrologic precision with macro dimensions. This method therefore makes it possible to construct structures of aspect ratios that normally can only be achieved by EDM (electric discharge machining) of metals. These advances in micromachined silicon fabrication technology permits executing classical designs for the XY stage 52 to provide metrologic quality.